Aluminum deposition from an anhydrous fusible salt electrolyte



United States Patent 3,236,751 ALUMINUM DEPGSITION FROM AN ANHYDROUS FUSIBLE SALT ELECTROLYTE Kuninori Azuma, Nara-ken, Japan, assignor to Matsushita Electric Industrial (10., Ltd, Osaka, Japan, a corporation of Japan No Drawing. Filed May 17, 1962, Ser. No. 195,422 Claims priority, application Japan, May 19, 1961, 36/173342 4 Claims. (Cl. 204-39) This invention relates to an anhydrous fusible salt electrolyte consisting of 60-85% by weight of aluminum chloride as principal agent and 25% by weight of alkaline earth metal chloride with or without alkali metal chloride and not more than 0.5% by weight of an alkali metal borofiuoride, as additives. This anhydrous fusible salt electrolyte is useful for plating aluminum on iron material with increasing service life of the electrolyte.

The known anhydrous fusible salt electrolyte suited for aluminum-electroplating is a binary system comprising 60-80% by weight of aluminum chloride as principal agent in admixture with sodium chloride or potassium chloride as additive. However, the known electrolyte as mentioned above can not provide satisfactory results. In order to obtain more improved results for desirable electroplating, another electrolyte which comprises 60- 90% of aluminum chloride, 9.539.5% of alkali metal chloride and 0.5-7% of alkali metal fluoride has been proposed. However, the resulting electroplating from the alkali metal fluoride-loaded electrolyte is somewhat hard. Such electroplating can show good luster when it is buifed, but the electroplating requires a considerably longer time for a determined thickness of the plating. This means that the electrolyte concerned is unsuitable for commercial scale production on account of the lower line speed. Furthermore, aging of the electrolyte is rapid, and the electrolyte tends to form a black sludgy impure coating on anodic aluminum plate thereby preventing circulation of electric current.

Generally speaking, aluminum plating should preferably be 7n-10 in thickness. Aluminum plating of less than 4 2 thick is insufficient in corrosion resistance and it is likely to expose a substrate when the plating is finished by buifing. Desirable plating should have a thickness of more than 7 after electroplating for a short period and it also should have proper hardness so as to make mirror polishing possible.

In order to overcome the drawbacks of the prior art, the present invention is accomplished by providing an electrolyte composition containing As principal agent:

Aluminum chloride percent 60-85 As additives:

Alkaline earth metal chloride, e.g., strontium chloride and barium chloride, together with or without alkali metal chloride, e.g., sodium chloride and potassium chloride percent by wt 10-25 Alkali metal borofluorides, e.g., sodium borofluoride, potassium borofluoride and the like "percent by wt j 0.5

The above-indicated components are essential for the practice of the invention. The addition of less than 0.4% of lithium chloride is preferable to impart activity to the electrolyte of the invention and to secure good circulation of electricity. Lead chloride can be added in an amount of less than 0.4% to have remarkably increased thickness of the resulting plating.

The amount of aluminum chloride which is the aluminum source of the aluminum-coating is preferably 60- by weight. The alkaline earth metal chloride is added to the electrolyte bath to prevent generation of metallic aluminum fog during plating and to increase the electroplating efficiency and obtain a smooth plating film due to the presence of the alkaline earth metal cations whose potentials are lower than that of aluminum. The amount of the alkaline earth metal chloride is 10-25% by weight, in which alkali metal chloride may be incor porated. The aluminum chloride and alkaline earth metal chloride form a double salt which is effective for the electrolyte bath and helps to produce a smooth plating surface.

The metal borofluorides in the electrolyte bath serve to prevent oxidation of aluminum and to decompose aluminum oxide formed, with the result of favoring circulation of electricity and of producing a dense and smooth plating surface. The borofluorides should be added in an amount of not more than 0.5% by weight to 60-85% by weight of aluminum chloride. If the amount of the borofluorides is more, metal fog is produced, current emciency is decreased, and the appearance of the plated meta-l is not good and corrosion of the plated metal is seen by means of the metal fog.

The electrolyte of the invention can be kept in completely liquid state at C. For plating in industrial scale operation, a temperature of -160 C. is suitable. The preferred electric current density may be 2-4 A./dm. and under such conditions, plating with a thickness of 7-10,LL or 15-20,:r can be obtained after a plating period of 2-5 min. or 10-15 min, respectively.

Now one embodiment of the invention will be detailed hereinafter.

Highly pure aluminum as anode and iron metal as cathode are dipped into the fused electrolyte composition at 145-160 C. Electroplating is carried out at 2-4 A./dm. at 2-5 v. for 2-5 minutes. Aluminum coat of 7-10 1. thickness is plated on cathodic iron. The coat applied on the iron material is smooth and shiny. With respect to thickness and hardness of the coat, the result obtained is satisfactory. Aging of the electrolyte is remarkably prevented.

According to one of conventional methods, electroplating is conducted in a bath comprising 9.5-39.5% by Weight of alkali metal chloride, 60-90% by weight of aluminum chloride and 0.5-7% by weight of alkali metal fluoride at a temperature of 500 F., surface speed of 500 ft./min. and current density of 250 ampere/ft? The bath temperature of said process is higher than that of the present process and heat control in the former way is more complicated. The thickness of plated metal per unit time in the case of the known method is 2-3 1. less than that of the present process of the invention, the final thickness of the coat according to the known method being 3-4, while that of the invention is 7-10 These advantages are due to the addition of the alkaline earth metal chlorides. The following are the typical illustrations of the electrolyte compositions of the invention.

Example 1 Principal agent: Percent Aluminum chloride 78 Additives:

Barium chloride l3 Strontium chloride 8.6 Sodium borofluoride 0.4

3 Example 2 Principal agent: Percent Aluminum chloride 75 Additives:

Sodium chloride 10 Barium chloride 8 Strontium chloride 6.6 Potassium borofiuoride 0.2 Lithium chloride 0.2

100.0 Example 3 Principal agent:

Aluminum chloride 80 Additives:

Barium chloride 12 Strontium chloride 7.5 Sodium borofiuoride 0.3 Lead chloride 0.2

In Example 3, the lead chloride present is in an amount of less than 0.4%, and no entrainment of lead in the aluminum plating is observed. Accordingly, the plated article obtained by using the electrolyte of Example 3 can be used as a cooking utensil or the like without any health hazard.

All the electrolytes of Examples 1 to 3 can provide smooth and shiny aluminum plating, with suflicient thickness and hardness.

To show utility of the present invention, several applications of the aluminum-plated article obtained by the invention will be given hereinunder.

One side of the aluminum-plated thin steel plate of the invention is electropolished and the other side is coated with a coloring paint. The plate is processed to form a blind of which the electropolished surface is faced outdoors and the painted surface faces indoors. This blind has a good sunlight-insulating effect with increased service life.

On the top surface of an engine valve, aluminum plating is applied according to the present invention. Heat resistance as well as service life is remarkably increased.

On the surface of a thermo-wire, aluminum plating is applied according to the present invention, thereby to have increased service life of the wire.

Steel wire, aluminum-plated according to the invention, is used to prepare a cage for poultry. Spot welding is allowable and the cage thus prepared shows increased mechanical strength and corrosion resistance. It is also possible to stain the steel wire with red color paint. Redcolored environment serves to have egg-laying increased.

What I claim is:

1. An anhydrous fused salt electrolyte composition suitable for aluminum plating consisting essentially of 60 to 85 percent by weight of aluminum chloride, 10 to 25 percent by weight of alkaline earth metal chloride and an alkali metal borofiuoride in an amount not exceeding 0.5 percent.

2. An anhydrous fused salt electrolyte composition as claimed in claim 1 wherein the alkaline earth metal chloride is a mixture of barium chloride and strontium chloride.

3. An anhydrous fused salt electrolyte composition as claimed in claim 1 which includes lithium chloride in an amount not more than 0.4 percent.

4. An anhydrous fused salt electrolyte composition as 0 claimed in claim 1 which includes lead chloride in an amount not more than 0.4 percent.

References Cited by the Examiner UNITED STATES PATENTS 1,709,759 4/1929 Weber et a1 20467 1,927,772 9/1933 Chittum 204-39 2,762,764 9/1956 Owen 20439 2,873,233 2/1959 Schnable 20439 2,919,234 12/1959 Slatin 20467 2,935,454 5/1960 Tokumoto 20439 2,971,899 2/1961 Hanink et a1. 20439 3,103,472 9/1963 Slatin 20467 JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

G. KAPLAN, Assistant Examiner. 

1. AN ANHYDROUS FUSED SALT ELECTGROLYTE COMPOSITION SUITABLE FOR ALUMINUM PLATING CONSISTING ESSENTIALLY OF 60 TO 85 PERCENT BY WEIGHT OF ALUMINUM CHLORIDE, 10 TO 25 PERCENT BY WEIGHT OF ALKALINE EARTH METAL CHLORIDE AND AN ALKALI METAL BOROFLUORIDE IN AN AMOUNT NOT EXCEEDING 0.5 PERCENT. 